[Leetcode]_10 Regular Expression Matching

1

p.empty()   &&  s.empty() == true   =>  true
&&  s.empty() == false  =>  false

s:     p:       finally:
a   => a*   =>  s, p.substr(2) or s.substr(1), p
=> .*   =>  s, p.substr(2) or s.substr(1), p
=> a    =>  s.substr(1), p.substr(1)
=> .    =>  s.substr(1), p.substr(1)
=> else =>  false

class Solution {
public:
bool isMatch(string s, string p) {
if( p.empty() )
return s.empty();
else if( p.length() > 1 && p[1] == '*' ) {
if( (s[0] == p[0] or p[0] == '.') and !s.empty() )
return (isMatch(s, p.substr(2)) or isMatch(s.substr(1), p));
return isMatch(s, p.substr(2));
}
else{
if( (s[0] == p[0] or p[0] == '.') and !s.empty() )
return isMatch(s.substr(1), p.substr(1));
}
return false;
}
};

2.DP

matchArray[i][j]: matchArray[i-1][j-1]
p[j-1] == '*'   =>  matchArray[i][j] = matchArray[i][j-2] || (s[i-1] == p[j-2] || p[j-2] == '.') && matchArray[i-1][j];
else     =>  matchArray[i][j] = matchArray[i-1][j-1] && (s[i-1] == p[j-1] || p[j-1] == '.');

class Solution {
public:
bool isMatch(string s, string p) {
int sLen = s.length(), pLen = p.length();
bool matchArray[1001][1001];
matchArray[0][0] = true;

for(int i = 1; i <= sLen; i++)
matchArray[i][0] = false;
for(int j = 1; j <= pLen; j++)
matchArray[0][j] = j > 1 && p[j-1] == '*' && matchArray[0][j-2];

for(int i = 1; i <= sLen; i++)  {
for(int j = 1; j <= pLen; j++)  {
if( p[j-1] == '*' )
matchArray[i][j] = matchArray[i][j-2] || (s[i-1] == p[j-2] || p[j-2] == '.') && matchArray[i-1][j];
else
matchArray[i][j] = matchArray[i-1][j-1] && (s[i-1] == p[j-1] || p[j-1] == '.');
}
}

return matchArray[sLen][pLen];
}
};